Seeing as their clever answer was given with Yaakov's consent, why was he angry at them when they carried it out?
Ramban #1: Yaakov gave his consent to the initial plan, 1 but not to Shimon and Levi, who took it upon themselves to avenge their sister's shame and to kill all the inhabitants of Shechem (an act that the brothers had not envisaged).
Ramban #2: Even if they had killed Shechem, Yaakov would not have been angry, since their plan had been formulated due the fact that 'they had defiled their sister' (as the Pasuk specifically states). It was the fact that they killed all the inhabitants of the town who had done them no harm, that aroused his ire.
How do we explain the fact that all the brothers agreed on the plan of action, yet it was only Shimon and Levi who carried it out, and Yaakov cursed only their anger?
Ramban: The other brothers thought that if they will circumcise, they can easily take Dinah and leave.
How could Yaakov agree that Dinah will marry a Nochri?
Ramban, R. Bechayei: The other brothers assumed that the townspeople would not consent to circumcise, and even if they would, they would just take Dinah and leave.
Perhaps they stipulated that he will convert (PF). Hadar Zekenim (to 34:25) says that they stipulated that the entire city will abandon idolatry!
QUESTIONS ON RASHI
Rashi writes that Bnei Yaakov spoke to them cleverly. What was so clever about their condition?
Seforno: They were convinced that either Shechem and Chamor, or the townspeople, 1 would never agree to circumcise, in which case they would take their sister and leave.
Ramban: They assumed that the townspeople would not consent to circumcise, and even if they would, they would easily be able to take Dinah and leave. 2
Hadar Zekenim: They said ''we would be willing to be like you - just one cannot return Orlah that was cut, so you must circumcise to be like us.'
Ohr ha'Chayim: They initially spoke harshly, to show that they were upset about what happened, and adamant about fulfillment of their stipulation. Had Shechem suspected that they are not sincere, he would have guarded himself!
Malbim: Their words objected only due to the Tum'ah of Bi'ah with an Arel - for which Shechem cannot be blamed for not knowing this - but not for the rape. Therefore, he thought that if they circumcise, they will consent to intermarry with them.
Bechor Shor: It was unreasonable that the townspeople would circumcise to enable Shechem to marry her. However, Shechem and Chamor convinced them that they will gain monetarily through this (refer to 34:25:6:1). Ha'amek Davar - Had they thought that the city will fulfill the stipulation, they would have stipulated about laws pertaining to marriage.
Which would be perfectly permissible, since Shechem defiled her, as the Pasuk specifically states. (EC)
Rashi writes: "'For he had defiled...' - The verse is telling us that this was not deceit, for he had defiled their sister Dinah." Perhaps this phrase is not the Torah's narrative, but a quote of Yaakov's sons' initial response to Shechem and Chamor?
Gur Aryeh: They certainly did not verbalize this at this point (saying 'You have defiled our sister!'), for then Shechem and Chamor would realize their intent to avenge the deed. Rather, the verse is telling us the justification for their response.
